Polyhydric alcohols, or polyols, are osmotically active organic molecules whose physiologic and pathophysiologic significance has only very recently come under investigation. These molecules are produced by the reduction of common sugars via the catalytic action of aldose reductase. It appears that the overproduction and accumulation of polyols such as sorbitol, the product of glucose reduction, in certain tissues, may be responsible for some of the dire consequences of chronic hyperglycemia, as in diabetes mellitus or hereditary galactosemia.